As integrated circuit geometries decrease and gate-counts increase, the power dissipated by integrated circuits is becoming so large that it is difficult to manage. Dissipating the heat is a problem. Supplying such large amounts of power to single chips is a problem. End users would find it very attractive if any particular chip could perform the same function at reduced power requirements.
Power is dissipated in CMOS devices as signals are charged and discharged between the two power rails. Power is a function of the frequency of these charge cycles and the current required for each charge cycle. The frequency cannot be reduced, as it is directly proportional to the processing power of the device. The required current is a function of the capacitance of the elements and the voltage difference between the power rails.
Presently, reductions in power consumption are achieved by reducing the core voltage of the device. Unfortunately, operating voltages cannot continue to be reduced at the same rate in which circuit densities are increasing. This is because the “noise” created by signal switching on data transmission lines lying in close proximity to each other tends to affect the reliability of the transmissions.
This noise originates from the inductive effects of a current moving in a transmission line. A moving current sets up a magnetic field around the conductor. The field consists of a series of concentric circles of influence which are measured in terms of their flux density. As the current in the conductor changes (e.g., as the binary signal being carried by the conductor changes state), the magnetic field associated with that changing current changes also.
Any charged particle existing within a changing magnetic field experiences a force of electromotance proportional to the rate of change of the flux density. This electromotance acting on the charged particle creates an electromotive potential on the particle. This is known as inductance. In the case of a charged particle existing within a conductor adjacent to the conductor setting up the changing magnetic field, all particles in the second conductor feel the electromotive potential. This potential is referred to as electromotive force, or EMF.
The size and nature of the EMF potential acting on the second conductor's signal is a function of the change in the first conductor's signal voltage and it's current. This EMF potential either adds to or subtracts from the ongoing device signal voltage (potential) which has been applied to the second conductor. Addition or subtraction of the two potentials depends on the relative directions of current flow and physical layout of the two conductors.
Thus, the current change caused by a change in binary status (0 to 1, or vice versa) in one line tends to create inductive voltage noise which propagates to adjacent lines. The noise is of a relatively small order of magnitude compared to the affected signal strength (around 1.0V) and the variation does not affect the integrity of the data transmitted. However, if much lower signal voltages were to be used, say on the order of 0.1V, the noise associated with binary status changes in one or more transmission lines could potentially affect the integrity of adjacent signals. This would tend to impact the relatively weaker voltage signals corrupting the data transmitted therein.
Presently, data transmission is accomplished via independent, single-ended signals which swing between the two power rails. Noise created as a result of these signals tends to propagate to adjacent data lines. If two identical signals on two adjacent lines are being transmitted simultaneously, their inductive noise effects tend to be additive. Conversely, if the same two signals are identical except for their polarity (i.e., one signal is driven by a positive voltage and the other a negative voltage) the inductive effects tend to be counteractive.
Problems occur when too many same-polarity signals are transmitted simultaneously, causing a spike in the induced voltages in adjacent lines. FIG. 1 indicates a theoretical prior art arrangement of data transmission lines 1 running between a pair of DSPs being in close proximity to clock signal lines 2. In this arrangement, the clock pulses are simultaneous on each line 2. The cumulative effect of such transmissions may create significant induction in the adjacent data transmission lines, potentially disrupting their signals. A balance in the polarity across most of the clock signals would tend to minimize the inductive effects. At the same time, if such reduced inductive effects could be achieved throughout a device, chip core voltages could be reduced without a corresponding increase in data corruption caused by uncontrolled induction. This lowered core voltage would, in turn reduce the power needed by such device.
Transmission voltages in today's electronic devices are maintained at higher levels than needed to transmit individual signals in an integrated circuit. This is because a high signal drive voltage is needed to differentiate an individual signal from the potentially detrimental noise associated with the great number of densely packed transmission lines in today's devices.